Preheat the oven to 180°C/350°F/gas 4. Line a 12-hole cupcake tray with paper cases.
On a chopping board, carefully cut the neck-end off the squash (save the rest for another day), trim away the stalk, then cut into rough chunks (there’s no need to peel it). Place in a food processor and whiz until finely chopped.
Crack in the eggs, then add the sugar, flour, baking powder, walnuts, cinnamon, extra virgin olive oil and a tiny pinch of salt. Whiz again until just combined – you may need to stop the processor after a couple of seconds and scrape the mix down from the sides with a spatula to help it mix evenly. Try not to overdo it with the mixing – you want to just combine everything and no more.
Use tablespoons to fill the paper cases three-quarters of the way up with mixture. Bake in the hot oven for 25 to 30 minutes, or until golden, risen and cooked through. To check if they’re done, stick a cocktail stick or skewer into the middle, remove it after 5 seconds and if it comes out clean they’re cooked; if it’s slightly wet, cook for a little longer.
Leave the cakes to cool slightly in the tray, then carefully place them onto a wire rack to cool completely.
Meanwhile, finely grate the lemon zest into a mixing bowl. Add the soured cream to the bowl and sift in the icing sugar. Halve the vanilla pod lengthways, scrape out the seeds and add them to the bowl. Gently mash half of the berries (if using) and add to the bowl, then mix well. Once cool, spoon the icing onto the cakes, then decorate with the remaining berries, if you like. Now, tuck in!